In researching the present invention, a number of patents have been discovered which relate generally, but not specifically, to the subject matter of the present invention. These patents include the following: U.S. Pat. Nos. 406,365; 2,774,684; 3,359,132; 3,535,157; 3,620,805; 3,672,043; 3,704,165; 3,776,771; 3,877,975; 3,924,794; 3,941,906; 4,277,518; 4,352,838; and 3,325,302.
Theodore Bostroem in U.S. Pat. No. 3,941,906 reveals a dip metallizing process which consists of passing an article to be metallized through a bath of molten metal such as lead and and conducting the article therefrom through a layer of molten coating metal such as zinc which is confined in a stack-like structure of a cross-sectional area which is a small fraction of the surface area of the heavy metal bath. The molten coating metal is continuously supplied to the stack through orifices so as to remove droplets of molten heavy metal adhering to the surface of the article. As will appear hereinbelow, this patent is not suggestive of submerged levelling knives which employ a surrounding liquid in order to remove excess material. Consequently, this patent relates somewhat in general to subject matter contained in the instant application but not specifically thereto.
David Shepard shows in U.S. Pat. No. 3,776,771 a provision for the application of an etch resist coating to plated holes in a printed circuit board. While this particular patent shows the use of horizontal rolls, and a horizontal workpiece, as well as the removal of excess material in connection therewith, the Shepard Patent does not show the use of levelling knives or superposed baths in the manner to be described in greater detail hereinbelow.
In U.S. Pat. No. 2,774,684, A. M. Fucinari discloses an invention which relates to dry film type drawing lubricants and methods and apparatus for applying compositions to sheet metal prior to forming articles therefrom. While this patent shows the use of series of horizontal rollers working with a horizontal workpiece, it shares little else with the techniques and apparatus of the present invention which is not directed to applying lubricants to sheet metal. As will be shown, this patent does not disclose the delimiting of a bath by means of horizontal rollers nor does it reveal many of the other features of the invention.
Travis Allen reveals in U.S. Pat. No. 3,924,794 the removal of excess or undesirable solder from printed circuit boards that may contain through-holes, conductors, connectors or the like. In this patent, printed circuit boards are covered with a liquid flux and contacted with a liquid solder and are removed from the solder and subsequently passed between hot gas jets which flow hot gas under pressure onto the boards to sweep off undesired solder, clear the holes, and leave an optimum thickness solder layer. This technique does not achieve the excellent results achieved with the levelling knives employed in conjunction with the present invention as will be discussed in greater detail hereinbelow.
In the McLain U.S. Pat. No. 3,704,165 is disclosed an invention relating to printed circuit boards which have been previously solder plated. The liquid involved has to be substantially above the melting point of the solder in order to be effective. This creates hot spots where there are large areas of copper resulting in thin coatings at the related points. The holes in the board also create a surface turbulence which further contributes to a non-uniform coating. In addition, the operation is extremely messy because of the fumes caused by the high operating temperature. This patent fails to reveal the use of submerged jets in the manner which will be discussed in greater detail hereinbelow.
The Schillke U.S. Pat. No. 4,277,518 employs a solder station. The use of submerged levelling knives is not suggested herein. Similarly, no use is made in this patent of rollers which both delimit the extent of a bath of solder assisting fluid while at the same time constituting the entry and exit for workpieces to be soldered. Consequently, there are distinct differences between the invention disclosed in this patent and the invention which will be discussed hereinbelow.
Other of the patents which have been mentioned hereinabove may relate to isolated details of the structure and technique to be discussed hereinafter. However, none of these patents, taken separately or in combination, reveals the inventive concept which will be found in the description which follows hereinbelow.